Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session V21: SpinPhoton Coupling in Semiconductor Quantum DotsFocus

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Sponsoring Units: GMAG DMP FIAP DCOMP Chair: Guido Burkard, Univ Konstanz Room: LACC 309 
Thursday, March 8, 2018 2:30PM  3:06PM 
V21.00001: Strongcoupling Cavity QED with Single Electron Charge and Spin Qubits in Silicon1 Invited Speaker: Xiao Mi Coherent coupling of single qubits to microwave photons provides a scalable pathway toward longrange qubitqubit entanglement. We first demonstrate strongcoupling between a single electron charge qubit in a gatedefined Si double quantum dot to a microwave photon in a superconducting cavity^{2}. Combining electricdipole interaction with spincharge hybridization in the presence of a magnetic field gradient, we also achieve the strongcoupling regime between a single electron spin and a single microwave photon^{3}. Spinphoton coupling rates up to 11 MHz are supported by the device architecture, exceeding direct magneticdipole coupling rates by five orders of magnitude. Furthermore, we demonstrate allelectric control and quantum nondemolition readout of the singlespin qubit using its dispersive interaction with the microwave cavity. These results allow for the construction of a largescale Si quantum processor with “alltoall” connectivity. 
Thursday, March 8, 2018 3:06PM  3:18PM 
V21.00002: Tunable Fewelectron Charge and Spin States in ParallelCoupled Quantum Dots in InAs Nanowires Malin Nilsson, Florinda Viñas Boström, Sebastian Lehmann, IJu Chen, Martin Leijnse, Kimberly Dick Thelander, Claes Thelander Highresolution spin spectroscopy is performed on parallelcoupled double quantum dots in the fewelectron regime. The system consist of a InAs nanowire of zinc blende crystal structure, having a single quantum dot (QD) epitaxially defined by two thin segments of wurtzite, acting as tunnel barriers (offset ~100 meV) [M. Nilsson et al, PRB 93, (2016)]. The small axial extension of the QD (<10 nm) leads to a strong quantum confinement and enables the QD to be fully depleted of electrons. By using pairs of local side gates and a global back gate the system can be tuned from one QD into parallel double QDs, for which we can control the populations down to the last electrons. The combination of hardwall barriers to source and drain, shallow interdot tunnel barriers, and very high singleQD excitation energies (up to 27 meV), allow an order of magnitude tuning of the strength for the first intramolecular bond. In addition, the consistently large g*factors (~9) facilitate detailed studies of the Bfield dependence of the 1 and 2electron states. Specifically, we find that it is possible to tune the magnitude of the Bfield induced singlettriplet anticrossing by changing the interdot tunnel coupling. We model the experimental data using a simple fewelectron Hamiltonian. 
Thursday, March 8, 2018 3:18PM  3:30PM 
V21.00003: InputOutput Theory for SpinPhoton Coupling in Si Double Quantum Dots Monica Benito, Xiao Mi, Jacob Taylor, Jason Petta, Guido Burkard The interaction of qubits via microwave frequency photons enables longdistance qubitqubit coupling and facilitates the realization of a largescale quantum processor. However, qubits based on electron spins in semiconductor quantum dots have proven challenging to couple to microwave photons. In this theoretical work [1] we show that a sizable coupling for a single electron spin is possible via spincharge hybridization using a magnetic field gradient in a silicon double quantum dot. Based on parameters already shown in recent experiments, we predict optimal working points to achieve a coherent spinphoton coupling. Our predictions are in good agreement with recent measurements [2] which demonstrate strong coupling with spinphoton coupling rates of more than 10 MHz. Furthermore, we employ inputoutput theory to identify observable signatures in the cavity output field, which can provide guidance to the experimental search for strong coupling in such systems and opens the way to cavitybased readout of the spin qubit. 
Thursday, March 8, 2018 3:30PM  3:42PM 
V21.00004: Spin orbit interaction in dynamics of silicon double quantum dots Ernesto Cota, Sergio Ulloa We present a theoretical study of the role of spinorbit interactions in a silicon double quantum dot. We propose that an accurate estimate of the strength of this interaction can be obtained through the study of the return probability of the double occupation singlet state in a magnetic field, as the system is gated dynamically across the relevant states in the low energy twoelectron manifold. LandauZenerStückelberg (LZS) type of processes involving appropriate control of voltage pulses across neighboring avoided crossings in the energy spectrum of the system are utilized to explore the system dynamics. Our description takes into account Zeeman splitting, intervalley mixing and spinorbit interaction present in the structure. Using a density matrix equation of motion approach, we carry out numerical calculations of the return probability of the double occupation singlet state. The analysis in terms of LZS theory allows the determination of the spinorbit coupling strength for different Zeeman splitting regimes. 
Thursday, March 8, 2018 3:42PM  3:54PM 
V21.00005: Optical Dependence of EDMR in Silicon Devices Lihuang Zhu, Kipp van Schooten, Mallory Guy, Chandrasekhar Ramanathan Electricallydetected magnetic resonance (EDMR) provides a highly sensitive method for reading out the state of donor spins in silicon. The technique relies on a spindependent recombination (SDR) process involving dopant spins that are coupled to interfacial defect spins near the Si/SiO_{2} interface. At cryogenic temperatures and lowdoping concentrations, optical excitation is used to generate the free carriers. We investigate the wavelength dependence of the EDMR signal in a Si:P device. With nearinfrared excitation we find that the EDMR signal primarily arises from donordefect pairs, while at higher photon energies there are significant additional contributions from defectdefect pairs. At longer wavelengths, the contribution of defect spins adjacent to the buried oxide layer also increased due to the increased penetration depth into the device. Careful tuning of the optical excitation energy allows us to control both the SDR dynamics and to characterize depthdependent features of the EDMR signal. 
Thursday, March 8, 2018 3:54PM  4:06PM 
V21.00006: Effect of Modified Periodic Waveforms on CurrentInduced Spin Polarization Measurements Joseph Iafrate, Davide Del Gaudio, Simon Huang, Rachel Goldman, Vanessa Sih Applying a timevarying periodic voltage to a semiconductor sample generates a currentinduced electron spin polarization (CISP). Using an ultrafast modelocked laser and lockin detection scheme, we measure CISP on a 500nm indium gallium arsenide epilayer (2.6% indium concentration) grown on a (001) gallium arsenide substrate via Faraday rotation and extract the spin generation rate. While the measured spin polarization initially increases linearly with electric field as observed in previous work, larger applied voltages lead to sample heating and a decreasing spin generation rate. We modify the applied voltage waveform to reduce heating, requiring that we add an extra data processing step to our measurement technique. We then recover the linear dependence of spin generation rate with electric field even at larger applied voltages. Future CISP studies can utilize this technique to investigate CISP under larger applied electric fields. 
Thursday, March 8, 2018 4:06PM  4:18PM 
V21.00007: Quantum Decoherence in Reduced Dimensions Meng Ye, Giulia Galli The electronic spin states of certain point defects in solid state materials have been shown to be promising quantum bits (qubit) for quantum information processes. Long spin coherence time is an important criterion to meet, in order to realize a qubit. In the presence of an external magnetic field and at low temperatures, decoherence originates mainly from the temporally fluctuating random magnetic field induced by nuclear spin flipflops. This process can be theoretically described by a spin Hamiltonian and approximately solved by a clustercorrelation expansion (CCE) method [1]. Recently, singlephoto emitters have been discovered in several reduced dimensional systems, such as 2D hexagonalBN. In this work, we used the CCE method to study the decoherence of a electron spin immersed in a nuclear spin bath as a function of dimensionality. We considered 3D bulk materials, thin films, and 2D materials. Our results shed light on the importance of dimensionality in determining the spin decoherence dynamics of spin defects and provide predictions about promising solidstate qubits in low dimensional systems. 
Thursday, March 8, 2018 4:18PM  4:30PM 
V21.00008: Spin effects in quench dynamics of quantum dots and molecules Ireneusz Weymann, Kacper Wrzesniewski We study the quench dynamics of correlated quantum dots and molecules attached to spinpolarized leads. We focus on the strong coupling regime, where electron correlations can give rise to the Kondo effect. We study the situations in which the quench is performed either in the coupling to external electrodes or in the position of the orbital level. The timeevolution of expectation values of local occupation and magnetization indicates a destructive role of the proximity induced exchange field on the Kondo state. For singlelevel quantum dots this takes place out of the particlehole symmetry point, while for molecules with larger spin detrimental effect of exchange field occurs in the whole Kondo regime. We also analyze the relevant time scales for the development of corresponding exchange fields and study their dependence on temperature, coupling strength and position of orbital level. The analysis is done with the aid of the timedependent densitymatrix numerical renormalization group method. 
Thursday, March 8, 2018 4:30PM  4:42PM 
V21.00009: Designing defectbased qubit candidates in widegap binary semiconductors Hosung Seo, He Ma, Marco Govoni, Giulia Galli The development of novel quantum bits is key to extend the scope of solidstate quantum information science and technology. Here, using firstprinciples calculations, we propose that large metal ion  vacancy complexes are promising qubit candidates in two binary crystals: 4HSiC and wAlN. In particular, we found that the formation of neutral Hf and Zrvacancy complexes is energetically favorable in both solids; these defects have spintriplet ground states and electronic structures similar to those of the diamond NV center and the SiC divacancy. Interestingly, they exhibit different spinstrain coupling characteristics, and the nature of heavy metal ions may ensure stability against defect diffusion. In order to support future experimental identification of the proposed defects, we report predictions of their optical zerophonon line, zerofield splitting and hyperfine parameters. The defect design concept identified here may be generalized to other binary semiconductors to facilitate the exploration of new solidstate qubits. 
Thursday, March 8, 2018 4:42PM  4:54PM 
V21.00010: Toward ultrafast spin lasers? Gaofeng Xu, Nils Gerhardt, Igor Zutic Injecting spinpolarized carriers in semiconductor lasers provides operating principles for room temperature spintronic devices not limited to magnetoresistance. Important steadystate properties have been demonstrated in these lasers, including threshold reduction [1] and spin amplification [2]. However, their main advantage is dynamical operation, predicted to have an enhanced modulation bandwidth, improved switching, and faster operation than the conventional lasers (with spinunpolarized carriers) [35]. As these predictions are being verified [6,7], we provide a generalized description of spin lasers to interpret related experiments and understand the limiting factors for their operation. [1] J. Rudolph, et al., Appl. Phys. Lett. 87, 241117 (2005). [2] S. Iba, et al., Appl. Phys. Lett. 98, 081113 (2011). [3] P. E. Faria Junior, G. Xu, J. Lee, N. C. Gerhardt, G. M. Sipahi, and I. Zutic, Phys. Rev. B 92, 075311 (2015). [4] J. Lee, R. Oszwaldowski, C. Gothgen, and I. Zutic, Phys. Rev.B 85, 045314 (2012). [5] E. Wasner,S. Bearden, J. Lee and I. Zutic, Appl. Phys. Lett. 107, 082406 (2015). [6] M. Lindemann, T. Pusch, R. Michalzik, N. C. Gerhardt,and M. R. Hofmann, Appl. Phys. Lett. 108, 042404 (2016). [7] H. Hopfner, et al., Appl. Phys. Lett. 104, 022409 (2014). 
Thursday, March 8, 2018 4:54PM  5:06PM 
V21.00011: Probing the Coherent Spin Dynamics of a Magnetic Impurity in a IIIV Semiconductor Stephen McMillan, Nicholas Harmon, Michael Flatté Individual magnetic impurities or small collections of magnetic impurities in IIIV semiconductors can be identified via scanning tunneling microscopy (STM)[1,2], their exchange interaction can be measured [3], and they can have remarkably long spin coherence times[4]. Coherent spin dynamics, however, has only been probed in ensemble measurements [4,5]. We describe an approach to explore this coherent spin dynamics through lowfield magnetoresistance involving a spinpolarized STM contact. Measurements of the spin coherence time and the local hyperfine interaction should be feasible. We also describe the effect of exchange interactions between magnetic impurities on the magnetoresistance. 
Thursday, March 8, 2018 5:06PM  5:18PM 
V21.00012: Optical spectral weight for the magnetooptical conductivity of topological spintronic semiconductors Zhou Li The Fermi velocity (v_{F}) associated with the spinorbit coupling is two orders of magnitude smaller for spintronic semiconductors than it is for topological insulators. Both families can be treated with the same Hamiltonian which contains a relativistic (Dirac) linear in momentum term proportional to v_{F} and a nonrelativistic quadratic contribution with Schrödinger mass (m). We find that the ac dynamic longitudinal and transverse (Hall) magnetoconductivities are strongly dependent on the size of vF. When the Dirac fermi velocity is small, the absorption background provided by the interband optical transitions is finite only over a very limited range of photon energies as compared with topological insulators. Its onset depends on the value of the chemical potential (μ) and on the magnetic field (B), as does its upper cutoff. Within this limited range its magnitude is, however, constant and has the same magnitude of e^{2}π/(8h) as is found in topological insulators and also in graphene noting a difference in degeneracy factor. 
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